1. Field of the Invention
The invention relates to an amplifier circuit, and more particularly to a high-efficiency amplifier circuit having an impedance matching circuit for high-harmonic signal components.
2. Description of the Related Art
It is important for a system using a transistor amplifier therein to enhance the electric power efficiency of the transistor amplifier. In particular, the enhancement in electric power efficiency is significantly needed for a portable system operating on a battery, as well as a satellite/space communication system. It is well known in the art that a transistor operating under a class-B biasing condition produces at an output node thereof a current that includes only even-high-harmonic components except for a fundamental wave component. Consequently, if such an arrangement would be made so that a voltage derived at the output node includes only odd-high-harmonic components, no power loss would take place in higher harmonics. A amplifier circuit having high-efficiency is thus derived.
In more detail, by designing a load impedance for even higher harmonics to be zero (0), no even-high-harmonic voltage component appears at the output node of the transistor. On the other hand, a load impedance for odd higher harmonics is designed to be large, preferably infinite, to obtain a voltage waveform. This voltage waveform is, however, not generated based upon the fundamental wave component, and not upon odd-high-harmonic current components, because the transistor produces the currents including only even-high-harmonic components as mentioned above. Thus, in order to enhance the power efficiency, such an impedance matching circuit is required in a transistor amplifier circuit that achieves in impedance matching with the fundamental wave component, an open state for odd higher harmonics and a short-circuit stage for even higher harmonics.
Based on the above mentioned technical concept, a transistor amplifier circuit as shown in FIG. 1 has been proposed. This amplifier circuit is disclosed in Japanese Unexamined Patent Public Disclosure No. 64-44611. In FIG. 1, an output terminal 34 of a transistor 31 is connected to one end of a line 33 having a one-eighth wavelength (.lambda./8) for a fundamental wave. This line 33 has the other end made open to thereby provide a quarter wavelength (.lambda./4) for a second high-harmonic component. The short-circuit state for the second high-harmonic component is thus performed. Since the one-eighth wavelength line 33 for the fundamental wave makes a capacitive stub, an output impedance of the transistor 31 is decreased lower than an intrinsic value thereof. Therefore, a matching circuit 32 is provided to match the output impedance thus decreased with a characteristic impedance Z.sub.0 of a transmission line 35.
However, the transistor 31 includes, in general, a plurality of unit transistors connected in parallel to one another for the purpose of gaining current capacity. Each of the unit transistors has a small output impedance, so that the output impedance of the transistor 31 is inherently small. Such a small impedance is further lowered by the capacitive stub by the line 33, as disclosed before. For this reason, in spite of providing the matching circuit 32, the power loss is increased remarkably due to the resistances thereof. Moreover, the impedance matching cannot be carried out over a wide band.